Tungsten and method of manufacturing the same



1931- w. B. GERO ET AL 1,826,514

TUNGSTEN AND METHOD OF MANUFACTURING THE SAME;-

Filed Nov. 26, 1926 4 Sheets-Sheet 1 FiChZ. l NTOR M56604 .Vavenport WM0 ATTORNEY Oct. 6, 1931.

w. B. GERO ETAL TUNGSTEN AND METHOD OF MANUFACTURING THE SAME Filed Nov.26, 1926 4 Sheets-Sheet 2 INVENTOR M5. 6ro d E. Uavpnport ATTORN EY Oct.6,1931.

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INVENTOR M5. 6era 6 5. Va ven orf ATTORNEY ()cf. 6, 1931. w. B. GEROETAL TUNGSTEN AND METHOD OF MANUFACTURING THE SAME Filed Nov. 26, 1926'4 Sheets-Shee t 4 FicT.9.

I INVENTOR MB. Gero a l. Davenport BY M/ XA'IZTORNEY- Patented Oct. 6,19 31 UNITED STATES PATENT] OFFICE WILLIAM BENJAMIN GERO AND EDMUNDSHARINGTON DAVENPORT, O1 BLOOMFIELD, NEW JERSEY, ASSIGNOBS TOWESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA TUNGS'IEN -ANDMETHOD OF MANUFACTURING THE SAME Application filed November 26, 1928.Serial No. 150,763.

This invention relates to filaments for incandescent lamps, power andradio tubes and the like, and more particularly to a filament of arefractory metal, such as tungsten, which 6 may be burned undervibration or shock conditions without detrimental sagging or offsetting.

The concentrated type of filament formed by coiling a fine wire oftungsten or similar 10 material into a small helix is preferred in manytypes of lamps and is found more desirable generally for certain typesof meandescent lamps and similar devices which employ a gaseousenvironment.

Thoriated tungsten filaments, when employed in such lamps, are burned ata much highertemperature than in the vacuum type of lamp and have beenfound in the past to become less eflicient after a comparativel shortperiod of operation. This loss in e ficiency has been primarilyoccasioned by the filament sagging which results in the turns of thehelix being spread apart, thus permitting a more rapid cooling action totake place.

In order to prevent this phenomenon of sagging from occurring, thefilament has been improved to the extent that structurally, the crystalsare large and arranged in overlapping and interlocking relation. Afilament of this type is said to be resistant to sagging and is formedof substantially ure tungsten. However, when a lamp containing a non-sagfilament is burned under shock or vibratory conditions without certainconstructional features in the lamp to prevent the shock or vibrationfrom reaching the filament, the life of the coil filament is relativelyshort, due to sagging or other phenomena taking place.

It has been found that thoriated tungsten filaments which contain from 1to 1% of thoria, and which are extensively employed in vacuum lamps,resist vibration and shock much better than the non-sag type of filamentbut cannot be coiled into such small helices as the non-sag wire.Furthermore, when such filaments are coiled and burned at the'temperatures prevailing in gas-filled lamps, a slow progressive saggingtakes place as a result of the small crystal structure theretaining thecustomary percentages of thoria' (about 1%) which may be readily formedinto helices of small diameter and which possesses an exaggerated orlarge crystal structure similar to that prevalent in the non-sag type offilament. We have found that our filament is resistant to substantialsagging and offsetting under vibratory and shock conditions and is caable of being burned a commercially use ul life.

We have further found that thoriated tungsten filaments manufactured inaccordance with our rocess possesses the unique property of de ormationwhen cold even in extremely small diameter filament sizes. The fact thatthis property manifests itself is novel in filament manufacture andfurthermore enables one to produce exceedingly small sized coils ofthoriated tungsten wire without heating the wire during the coilingoperation. Filaments of this type are useful not only in the lamp art,but also find utility in the manufacture of radio tubes, particu larlythat type which employs as the electronemitting element a tungsten,thorium-actr vated filament, i. e., a thoriated-tungsten filament havinga so-called surface layer of thorium metal, the latter serving as thesource of electrons. By reason of our invention, it is now possible tomanufacture radio tubes of much smaller sizes and to employ in thevarious types of tubes a coiled filament which by merely regulating thepitch of the coils or spacing thereof determine the effective length ofthe filament so as to fit any given size tube. Such an adaptation ofcoiled thoriated filament wire contributes materially to thestandardization of various types of radio tubes.

Therefore, it is anobject of our invention to provide a wire of tungstenor similar refractory metal which can:be coiled into a very small helixand burned at high temperatures tungsten filaments so as to render thesame resistant to substantial sagging and offsetting when burned undervibratory and shoclgcon ditions.

A further object of our invention is to provide a method of developingin a thoriated tungsten filament a large crystal structure.

A still further object of our invention is to rovide a thoriatedtungsten filament in the orm of a small coil whlch may be increased inlength at will so as to provide different length filaments or cathodesfor a standard line of radio tubes.

Other objects of our invention will become apparent as the followingdescription is read in connection with the accompanying drawin s inwhich:

ig. 1 is a longitudinal sectional view of a recrystallized thoriatedtungsten filament illustrating the usual crystal structure-there- Fig. 2is a longiturinal sectional view of a recrystallized thoriated tungstenfilament treated in accordance with our invention and showing theexaggerated crystal structure obtained thereby;

Fig. 3 is a vertical sectional view of the annealing apparatus;

Fig. 4 is a graph showing a curve resulting from plotting the finishedmilligram weight as abscissae against the percentage difference betweenfinished milligram weight and selected or initial milligram weight asordinates;

Fig. 5 is a graph showing a curve obtained by plotting as abscissae thefinal diameter in mils of the filament against the size of the filamentin mils which would be selected for annealing as ordinates;

Fig. 6 is a vertical sectional view of a standard radio tube of the UVor UX199 type.

Fig. 7 is a vertical sectional view of a radio tube of the UV or UX199type employing a coiled thoriated tungsten filament manufactured inaccordance with our invention; Fig. 8 is a vertical sectional view of astandard radio tube of the UX201A type. Fig. 9 is a vertical sectionalview of a radio tube of the UX201A size employing a coiled thoriatedtungsten'filament manufactured in accordance with our, invention andillustrating the novel construction made possible b the employment ofsuch a filament; an

Fig. 10 is a vertical sectional view of a standard incandescent electriclamp employing a cold coiled thoriated tun sten filament of relativelsmall diameter which is resistant to shoe and vibration withoutsubstantial sagging or offsetting.

We have discovered that an exaggerated crystal structure may be obtainedin a thoriated tungsten filament which is comparable to that obtained bythe use of certain alkaline earth or alkali metal com ounds whenemployed in the production 0 what is commonly termed in the lam industryas non-sag wire. The discovery is the result of a large number ofexperiments conducted on thoriated tungsten filaments and a study oftheir crystal structure after the filaments have been subjected tovarious treatments.

In the course of our experiments and study of the crystal structure. wehave determined that there is a definite law governing the developmentof crystals in thoriated tungsten filaments. This law is that for adesired finished size of filament in which a large crystal structure isto be had, it is essential to anneal and work the wire a definiteamount, such treatments being performed with an initial size wireselected in accordance with the finished size desired.

We have taken thoriated tungsten wires varying in size and annealed thesame and then mechanically worked them to different sizes and afterheating the same to at least about the temperature at which rapid graingrowth takes place have obtained in such wires a cr stal structure whichis markedly different rom that ordinarily obtained in the same wire whenfabricated in accordance with the usual commercial processes.

Our invention briefly stated, resides in the discovery that a markedimprovement may be made in the propertiesof a regular thoriated tungstenfilament which enables the same to be coiled cold at extremely smallsizes and which causes the filament, when burned at elevatedtemperatures or when heated to at least about the temperature at whichrapid grain growth takes place, to have an exaggerated or a largeoverlapping crystal structure similar to that present in regular nonsagwire formed of substantially pure tungsten metal. These unusualproperties are developed in the filament by a specific mode of treatmentwhich latter, concisely stated, comprises first selecting a filament ofthoriated tungsten at a definite size larger than the final sizedesired, annealing the filament and mechanically working the same bydrawing through dies or otherwise to the desired final size. It has beenfound that the annealing treatment reduces the tensile strength of thefilament in such manner that it ma be readily deformed cold by bendingor ot erwise so that it may be formed into coils of small diameterwithout fracturing the filament. Furthermore, the inherent nature ofproperties of the filament are such that when the filament is heated toabout the temperature at which rapid grain growth takes place, anexaggerated crystal structure is formed. Althou h no single theoryadequately accounts for t e unique propertles imparted by our process tothoriated filaments, we believe it is a result of putting the wire in astate of critical strain, and that this condition is responsible for thevarious ihenomena exhibited by wires so treated. epeated experimentshave demonstated that thoriated tungsten filaments produced inaccordance with our invention possess certain inherent properties whichare not present in regular thoriated tungsten filaments.

In accordance with a specific embodiment of our invention a tungstenfilament containing from to 1 percent of thorium oxide, manufactured inaccordance with the usual standard processes in which the additivemateria'l is incorporated in the tungsten metal oxide prior toreduction, is so conditioned that instead of the resultant filamenthaving a fine crystal structure 11, Fig. 1, it has an exaggerated orlarge overlapping crystal structure 12, Fig. 2, similar to the crystalstructure present in the non-sag wire now in use and also possessing theruggedness and resistance to crystal growth after the crystals are onceformed which is a characteristic property of thoriated tungstenfilaments. This change in the crystal structure is effected by modifyingthe inherent characteristics or properties of the filament by suitableheattreatment at proper wire sizes followed-by a definite andpredetermined amount of mechanical working.

The selection of the wire at a definite size above the final sizeconstitutes an important discovery as we have determined from numerousexperiments that unless such procedure is followed the ultimate resultis not obtainable. The results of our experiments have been plotted andcurves obtained thereby which may be followed in practicing ourinvention. In Fig. 4, the curve A is obtained by plotting as ordinatesthe percentage difference between the final milligram weight and theinitial or annealing milligram wei ht of the wire against the finalmilligram weight as the abscissae. By milligram weight is meant theweight in milligrams of a 200 millimeter length of wire.

A simpler curve B and one more readily understood by the layman is thatshown in Fig. 5. In this figure the curve is obtained by plotting asordinates the annealing or initial size in mils of the wire against thefinal size in mils of the wire. For example, if it is desired to know atwhich size to anneal the wire in the case where the final size of wiredesired is 2 mils it is merely necessary to follow vertically theordinate corresponding to the 2 Having determined the initial orannealing size in the manner indicated, the wire 12 (Fig. 3) issubjected to the following process.

The selected size of wire on a spool or reel 13 is placed upon suitablesupports 14, and the free end thereof attached to a spool or reel 15mounted on suitable supports 16, the spool being rotated at a suitablespeed by a belt 17. The rate at which the wire is moved governs, inaddition to the temperatures used in heating the wire, the degree ofheat treatment given the wire.

In heat-treating the wire, we prefer first to subject the same to apreliminary heating in air or under oxidizing conditions, although thisstep is not essential and may be dispensed with. This preliminaryheating is accomplished by passing the wire through a preliminaryheating chamber 18, about twenty-five inches in length, havingelectrical contact cups 19 and 21 of mercury. These contact cups areconnected to any suitable source of electrical energy 22 by conductors23 and 24, a resistance 25 being connected in circuit to regulate thecurrent passing through the circuit. The heating current may be passedthrough the wire 12' when the latter makes contact with the mercury cupsand the resistance 25 is properly adjusted.

The wire is preferably heated in the preheating chamber to a dull redheat, or below, and this temperature is maintained constant throughoutthe passage of the wire through the preheater.

The wire is then directly passed through an annealing furnace 26, heatbeing applied by passage of a direct current of electricity through theWire. The current is applied through a circuit 27 connected with asource of energy 22 and resistance element 28, and mercury contact cups29 and 31, with which the wire being treated makes contact. Theresistance 28 is so adjusted as to maintain the temperature of the wireat about a white heat. We have found that with a distance of 6 inchesbetween the contacts 29 and 31 and a voltage between the contacts offrom 65 to 70 volts, a wire ranging from 9.67 to 10.02 milligrams per200 mm. in weight becomes satisfactorily annealed if the speed at whichthe wire is drawn through the box is about 21.5

meters per minute. A low tension of about 20 grams should be maintainedor just sufiicient tension to keep the wire taut. Throughout theannealing of the wire a constant stream of hydrogen is kept flowinthrough the annealer by means of the condults 32 and 33, the rate offlow being approximately 4 to 5 cubic feet per hour in an annealer 6inches between contacts.

\Ve have found that the anneal given the wire is satisfactory in thecited case as well as others i'F't-he tensile strength of the wire afterannealing has been reduced at least about thirty percent. The durationof the annealing treatment as well as the temperature at which theannealin is performed, may be varied with different sizes and characterof wire as long as the foregoing resultv as to diminution in tensilestrength is obtained. The wire when it comes from the annealer is brightand metallic in appearance.

The next step after the wire has been annealed is to mechanically workthe wire to the final size. This may be done in any one of several waysbut we prefer to first coat the wire with a suitable drawing lubricantand pass the same through diamond dies.

We have found that thoriated wire, treated in accordance with theabove-described method, is more readily coiled than similar wire whennot given such a treatment. Furthermore, such treatment has made itpossible to produce small coils from thoriated tungsten wire of lessthan a mil in diameter by winding the same about a mandrel without thewire being heated.

Numerous experiments and tests made by us have demonstrated that theinherent properties of the metal have been changed to such an extentthat the metal is capable of'being deformed without exhibiting anydetrimental cracks. Furthermore, 4 the improvement made is furtherindicated by the change in crystal structure which takes place when thewire is heated to at least about the temperature at which rapid graingrowth takes place. This crystal structure consists of exaggerated orlong overlapping crystals similar to that obtained in non-sag tungstenwire. Our filaments, however, possess the additional desirable propertyof ruggedness and resistance to further crystal changes after thedesired long, overlapping structure has been formed. We attribute thisresistance to further crystal growth to the equilibrium established.

- Our improvement has found utility in incandescent lamps which areburned under such conditions that the filament is-subject to vibrationand shock, and has enabled the production of relatively small lamps,such as the watt size, with coiled thoriated tungsten filament. A lampof this general nature is illustrated in Fig. 10, in which a nonsag andvibration resistant thoriated tungsten filament 34 is shown. It is to benoted that this lamp is of the usual lamp design and does not employ anyspecial form of mount in order to prevent the shocks from beingtransmitted to the filament.

. Another utility for which our invention is capable is illustrated inFigs. 6 through 9. In Fig. 6 a type of radio tube known as UV or UX-199is illustrated. The filament in such tubes is of straight or uncoiledthorium 35 activated tungsten. By reason of our filament the over-alldimensions of this design of tube can be materially reduced b merelysubstituting a coiled filament 36 (Fig. 7) for the straight filament, itbeing appreciated that the same length of wire in the form of a coiloccupies considerably less space linearly.

Heret-ofore, such a tube has not been possible,

owing to the inability of coiling a thoriated tungsten filament ofrelatively small diameter.

Furthermore, the present design of UV201-A radio tube which employs anuncoiled V-shaped filament 37 (Fig. 8) can be modified so that a tubecan be formed giving the same electron current by merely substituting acoiled tungsten filament 38 (Fig. 9), of shorter over-all length buthaving the same effective length from the electron emission standpoint.The tube may be further modified by reason of the employment of such afilament, by using a cylindrical envelope, plate and grid, similar tothe present design of UV-199. A standard thus be obtained.

Although a specific mode of treatment and particular ingredients-havebeen described, it is believed that those skilled in the art may modifythe same, however, such modifications are contemplated by us as comewithin the scope of our invention as defined by the appended claims.

lVhat is claimed is:

1. The method of treating drawn thoriated tungsten filament so that thesame is inherently capable of exaggerated grain growth, when incandescedto elevated temperatures, which comprises annealing the filament to awhite heat under slight tension at a definite size larger than the finalsize and then mechanically working the filament to the final size.

2. The method of rendering a thoriated tungsten filament inherentlycapable of exaggerated grain growth when heated to at least about thetemperature at which rapid grain growthtakes place which comprisesannealing under slight tension a selected size of drawn filament largerthan the desired final wire size but in definite relation thereto untilits tensile strength has been appreciably decreased and thenmechanically working the wire to the final size.

3. The method of forming a coil type incandescent lamp filament fromdrawn thori ated tungsten filament less than 2.0 mil diameter whichcomprises annealing said filament during the drawing operation at a sizelarger line of tubes maythan the desired finished size of said filamentbut in definite relation. thereto, to a tensile strength approximatelyper cent of the tensile strength prior to annealing, then continuing thedrawing operation to the desired finished size, and thereafter formingsaid coil without the application of heat.

4. The method of forming a coil type incandescent lamp filamentcomprised of tungsten containing approximately 1.0 per cent thoriumoxide having an enlarged elongated overlapping crystal structuresubstantially resistant to sag and ofi'setting under vibrationconditions, which comprises annealing said filament a predeterminedamount at a size larger than the desired finished size but in definiterelation thereto, drawing said filament to the desired finished size,forming said coil without the application of heat, and thereafterheating said filament to elevated temperatures approximating the zone ofrapid grain growth to form the desired enlarged crystal structuretherein.

5. The method of forming a shock and vibration resistant filamentcomprised of tungsten containing from .75 to 1.0 per cent thorium oxide,which comprises imparting to said filamentary body during the drawingprocess a predetermined amount of working y means of an annealing stepfollowed by a drawing step, said annealing step being applied at a wiresize in fixed size relation to the size obtained from said followingdrawing step, and thereafter forming said drawn filament in any desiredshape without the application of heat thereto, and after incorporatinginto a lamp, heating said filament at elevated temperaturessubstantially equal to the temperature of rapid grain growth of the coldworked metal body to form the desired elongated, overlapping crystalstructure therein.

In testimony whereof, we have hereunto subscribed our names this 22ndday of November 1926.

WILLIAM BENJAMIN GERO. EDMUND SHARINGTON DAVENPORT.

